Fluid Class Search Index Manager for Incremental Build

We are on CS 9.2, using Fluid Class Search.  As we are now processing new indexes for upcoming terms, I am trying to understand what puts an index into Pending status.  This is under Curriculum Management > Fluid Class Search > Class Index Manager for building incremental indexes.  I am including screenshots that show the Last Successful Index build and the Pending Indexes, as well as the Details for the terms in Pending Indexes.  What causes a class to be put into the Pending Index, and what is the meaning of Add versus Change in the details?

Thanks,

Janelle